For many high-speed applications, a 50Ω transmission line environment is used to carry signals from one point to another while preserving their characteristics. Typically, a driver circuit transmits the information signals to the transmission line for transmission to their destination. For digital communication applications, the key signal characteristics that are desirable to preserve are: pulse amplitude, fast rise and fall times, and low jitter.
In a transmission line environment, the output of the transmission line must be terminated with its characteristic impedance, generally 50Ω, in order to prevent signal reflections from the load back to the source or driver. This is called the load termination. If reflections do occur, the signal characteristics may be degraded. In systems operating at higher frequencies it becomes increasingly difficult to ensure that the load termination impedance is equal to the characteristic impedance at all frequencies of interest as parasitic inductances and capacitances begin to have a larger effect at the higher frequencies. This means that at high frequencies, there will be some reflections from the load back to the driver. In these cases, by terminating the driver end of the transmission line with its characteristic impedance, these reflections from the load will be absorbed in the source termination rather than being reflected back to the load where they would degrade the signal characteristics. This source termination is achieved by having the impedance looking back into the driver output be equal to the transmission line characteristic impedance for all frequencies of interest.
FIG. 1 shows a particular example of a transmission line environment 100 being used to generate the digital waveform 105 shown at the load. A driver circuit, shown within the dashed box 110, drives the transmission line TL1 with a series of pulses to generate a required digital waveform. TL1 represents the transmission line with a 50Ω characteristic impedance. It is terminated with a 50Ω resistor, Rlp. When a transmission line is terminated with its characteristic impedance, the impedance looking into the input of the transmission line is equal to the characteristic impedance. In FIG. 1, at the point labelled MODP, the impedance to the right looking into the transmission line is 50Ω, and the impedance looking to the left into the driver output is 50Ω(Rop) since the output impedance of Q1 is high. Rop acts as a source termination resistor.
The desired waveform at the load consists of a series of pulses to represent a serial stream of logic ‘1’s and logic ‘0’s. The amplitude of the pulses is Vsw, the swing voltage. For this particular application, it is also required that the pulse waveform be offset below GND by the offset voltage, Vos. A “differential pair current switch” driver generates the swing voltage. Using appropriate drive signals at the base terminals of Q1 and Q2, the current IMOD is either all flowing in Q1 or all flowing in Q2. Assuming for now that the current IBIAS is zero, when IMOD is flowing in Q2, no current flows in the parallel combination of Rop and Rlp and hence, the voltage at the load, Vl, is equal to GND. When all of IMOD is flowing in Q1, IMOD flows in the parallel combination of Rop and Rlp. In this case, Vl is as described in eqn. 1.Vl=IMOD×50 Ω∥50Ω=IMOD×25 Ω  Eqn. 1
The swing voltage is the difference in the voltage at the load between the case when IMOD is switched to Q2 and when IMOD is switched to Q1:Vsw=IMOD×25Ω  Eqn. 2
From Eqn. 2, it can be seen that controlling the DC current IMOD controls the amplitude of the pulse waveform.
The DC IBIAS current is used to set the amplitude of the offset voltage Vos. IBIAS flows in the parallel combination of Rop and Rlp offsetting the pulse waveform below GND.Vos=IBIAS×25Ω  Eqn. 3.
The example shown in FIG. 1 is of a single-ended output driver and as such the complementary side of the differential pair current switch should be terminated with Rln to balance the load seen on both sides of the current switch. This resistor may be provided either internally on, or coupled externally to, the driver circuit. The provision of this resistor ensures that the resistance at the collectors of both Q1 and Q2 is the same. In FIG. 1, it can be seen that the resistance at the collector of Q1, being Rop in parallel with Rlp, is equal to the resistance of at the collector of Q2, being Ron in parallel with Rln, as required. Since the currents and load resistances are the same, the voltage at both collectors will also be the same, which is important for waveform integrity at the output. However, it will be appreciated that the degree of balancing of the loads can be traded off against some other aspect, so that in some cases Rln may not be the same value as Rlp.
The external inductor, L1, is used to decouple the capacitance of the IBIAS current source from the MODP output. The MODP is the high frequency output and if the IBIAS current source was directly connected to it, the pulse characteristics would be degraded by the presence of extra parasitic capacitance. The main degradation would be an increase in the rise and fall times.
One typical use or implementation of such driver circuits is in the digital transmission of signals over a fiber optic link. Such transmission consists of switching of laser light between a low level and a high level. Optical transmitters can be divided into two categories, those that use direct modulation, and those that use external modulation. With direct modulation, the laser light is switched between the high and low levels by switching the laser current between two levels. With external modulation, the laser is operated at a fixed optical power level and another device called an external modulator is used to modify the intensity of the laser light, thereby switching the transmitted optical output power between two levels. An EA (Electro-Absorption) modulator is one type of external modulator.
FIG. 2 shows a typical EA modulator electrical to optical transfer function and the drive signal that is applied to the modulator. The switching signal has an amplitude Vsw, the swing voltage. It is offset from ground by Vos, the bias offset voltage. When the input data is “0” both the swing and offset voltages are applied to the modulator. When the input data is “1” only the bias offset voltage is applied to the modulator. Typical swing voltages are in the 0.5V to 2.5V range, and typical bias offset voltages are between 0.25V and 1.2V.
FIG. 3 shows a combination of the driver circuit of FIG. 1 together with an EA modulator 300 as would typically be implemented in a fiber-optic transmitter application. As will be seen from FIG. 3, the 50Ω load, Rlp, that was present in FIG. 1 has been replaced by the EA modulator circuitry. EA modulator modules are designed for use in a 50Ω transmission line environment. The Electro-Absorption Modulated Laser (EML) has a 50Ω termination 305 integrated in the module. This termination resistor is in parallel with the modulator, which in FIG. 3 is represented by a reverse biased photo-diode 310. The reverse biased diode has a higher impedance than 50Ω, so the parallel combination has a resistance close enough to 50Ω to provide a good termination of the transmission line. With the appropriate drive voltage across the modulator, the light from a CW laser 315 is switched between low and high power levels to represent the digital data stream. It will be apparent from FIG. 3 that the optical output 320 from the laser 315 is modulated by the modulator prior to its transmission along the optical fiber 325.
It will be appreciated that, although the driver application described above is for a specific fiber optic transmitter application, as far as the driver is concerned, its load is a terminated transmission line. The same driver topology could be used in other applications where a similar type of drive signal is required and the load is a terminated transmission line.
It will be appreciated that, in order to create the required offset which is required for this controlled impedance application, the circuitry of FIGS. 1 and 3 requires a separate IBIAS current to generate the offset. The specific offset is established by IBIAS flowing in Rop and Rlp. The requirement for a specific source of this IBIAS current requires a separate pin and external inductor to prevent the capacitance of the IBIAS current source from reducing the speed, i.e. the rise/fall times. It will be further understood that an individual source of IBIAS increases the power requirements of the driver circuit.
There is therefore a need for an alternative driver implementation that will utilise less pins that the prior art configurations and have reduced power requirements.